1. Field of the Invention
The present invention relates to a switching power supply and more specifically relates to the pulse width modulation (PWM) controller of the switching power supply.
2. Background of the Invention
With the advantage of high efficiency, smaller size and lighter weight, switching mode power supplies have been widely used in electronic appliances, computers, etc. A typical switching mode power supply generally includes a PWM controller, a power MOSFET, a transformer and a feedback control circuit. The feedback control circuit is used to sense the output voltage and/or the output current in the secondary side of the power supply, and then connect to the PWM controller through an isolated device such as optical-coupler to achieve the feedback loop. FIG. 1 shows a traditional flyback power supply. A capacitor 220 connected to a PWM controller 100 is charged via a resistor 210. The PWM controller 100 is started up once its supply voltage Vcc is higher than the start-threshold voltage. When the PWM controller 100 starts to operate, it will output a PWM signal to drive a MOSFET 300 and a transformer 400, meanwhile its supply voltage VCC will be supplied by the auxiliary winding of the transformer 400 through a rectifier 230. A resistor 240 converts the switching current of the transformer 400 into voltage signal for PWM control and over-power protection. The feedback voltage VFB is derived from the output of an optical-coupler 250. The output voltage conducted through a resistor 290 and a Zener voltage of the Zener diode 280 drive the input of the optical-coupler 250 to form the feedback-loop. Through the PWM controller 100 the voltage VFB determines the on-time (TON) of the PWM signal and decides the output power. A transistor 260 associates with a current-sense resistor 270 and determines the maximum output current. As the output current increases and the voltage across the current-sense resistor 270 exceeds the junction voltage of the transistor 260 such as 0.7 V, the transistor 260 will be turned on to reduce the on-time(TON) of the PWM signal through decreasing the feedback voltage VFB and thus clamping the output current of the power supply as a constant.
Although the forgoing circuit is able to regulate output voltage and output current, it is difficult to shrink the power supply without eliminating the optical-coupler and secondary feedback control-circuit. Furthermore the current-sense resistor for the constant current output increases the power consumption of the power supply. According to the present invention, a primary side control eliminates the need of optical-coupler and secondary feedback control-circuit, and therefore reduces the device counts and the size of the power supplies, and so saves cost. Additionally, because the current-sense resistor is not necessary for the constant current output, the efficiency of the power supply is thus improved.
The present invention provides a technique to control the output voltage as well as the output current without the need of the feedback circuit in the secondary side of the power supply. The PWM controller indirectly senses the output voltage through its supply voltage, which is supplied by the auxiliary winding of the transformer. A feedback synthesizer is designed to generate a feedback current proportional to the variation of the supply voltage. Since the supply voltage produced by the auxiliary winding is correlated with the output voltage of the power supply, as the output voltage in the secondary side varies due to the variation of the load, this will result in a proportional variation in the auxiliary winding as well. However, the variation of the current flowing through the auxiliary winding creates different voltage drops and greatly affects the accuracy of the detection for the output voltage. In order to improve the regulation, an adaptive load is operated in the form of current, which is varied inversely proportional to the feedback current, which therefore achieves a constant supply current flowing through the path of the auxiliary winding. Consequently, the voltage drops in the auxiliary winding path will not affect the detection of output voltage. Furthermore, a programmable power limiter in the PWM controller controls the power delivered from the primary side of the transformer to the output of the power supply. The threshold of the power limit is varied in accordance with the change of output voltage. Since the output power is the function of the output voltage of the power supply, a constant current output is realized when the output current of the power supply is greater than a maximum value.
Advantageously, the PWM controller can regulate the output voltage and provide a constant current output through the primary side control, which eliminates the need of a feedback control-circuit in the secondary side. Therefore the device counts, the size of the power supply and the cost are greatly reduced. It is to be understood that both the foregoing general descriptions and the following detail descriptions are exemplary, and are intended to provide further explanation of the invention as claimed.